Method of screening cell death inhibitor

ABSTRACT

There is provided a method of screening for substance that inhibits cell death induced by activation of PARP, particularly a substance that is useful as a therapeutic and/or preventive agent for rheumatoid arthritis, neuronal death at the time of cerebral ischemia, cell death of the heart after myocardial infarction reperfusion, autoimmune destruction of β-cells of pancreatic islets of Langerhans, cell death after shock, or inflammatory reaction by immunocyte death. Also provided are a novel protein and a novel gene encoding the same. The aforementioned screening method comprises a step of allowing a test substance to contact a cell expressing an LTRPC2 protein under a conditions such that the LTRPC2 protein can be activated, and a step of analyzing inhibition of LTRPC2 protein activation. The aforementioned novel protein is a rat or mouse LTRPC2 protein, and the aforementioned novel gene is a rat or mouse LTRPC2 gene.

This is a National Stage of International Application No. PCT/JP02/08128, filed Aug. 8, 2002.

TECHNICAL FIELD

This invention relates to a method for screening a cell death inhibitor. Also, the present invention relates to novel rat LTRPC2 and mouse LTRPC2.

BACKGROUND OF THE INVENTION

Poly-ADP-ribose polymerase (PARP) existing in the nucleus is an enzyme which is activated by DNA degradation caused by inducible nitrogen oxide, oxygen radical and the like and thereby effects addition of the ADP ribose moiety of nicotinamide adenine dinucleotide (NAD) to nuclear protein and DNA. That is, a DNA damage activates PARP to cause addition of a large amount of ADP ribose to histone and PARP itself while consuming ATP. It is considered that intracellular NAD and ATP are reduced as the result to cause cell death due to intracellular energy depletion (Zhang, J. and Snyder, S. H., Science, 263, 687–689, 1995). Five genes coding for the PARP protein have so far been reported, and it is considered that the PARP activity is expressed through functioning of these genes. Among these genes, PARP-1 gene is the gene analyzed in most detail and is considered to be a gene having the most high contribution to the cell death accompanied by the activation of PARP protein (Smith, S., Trends Biochem. Sci., 26174–179, 2001).

It has been reported that NAD depletion by a DNA damage in cells of pancreatic islets of Langerhans does not occur in a PARP gene-deleted mutant mouse (Heller, B. et al., J. Biol. Chem., 270, 11176–11180, 1995). In addition, according to another reference, it has been revealed that the NAD depletion by a DNA damage does not occur also in cerebral cortex neuron in the aforementioned mutant mouse, and it has been reported that the cerebral infarction range accompanied by transient cerebral ischemia is significantly reduced in this mouse (Eliasson, M. J. et al., Nature Med., 3, 1089–1095, 1997). Thus, it is known that the cell death induced by the activation of PARP causes serious diseases in tissues of the living body. Also known as diseases in which the cell death induced by the activation of PARP is concerned include neuronal death at the time of cerebral ischemia, cell death of the heart after myocardial infarction reperfusion, autoimmune destruction of β-cells of pancreatic islets of Langerhans in type I diabetes mellitus, cell death after shock, inflammatory reaction by immunocyte death, and rheumatoid arthritis as an autoimmune disease which onsets by the generation of an abnormality in immune functions (Eliasson, M. J. et al., Nature Med., 3, 1089–1095, 1997; Zingarelli, B., Salzman, A. L. and Szabo, C., Circ. Res., 83, 85–94, 1998; Burkart, V. et al., Nature Med., 5, 314–319, 1999; Pieper, A. A. et al., Proc. Natl. Acad. Sci., 96, 3059–3064, 1997; Szabo, C., Cuzzocrea, S., Zingarelli, B., O' Connor, M. and Saltzman, A. L., J. Clin. Invest., 100, 723–735, 1997; Oliver F. J. et al., EMBO J., 18, 4446–4454, 1999; Szabo, C. et al., Proc. Natl. Acad. Sci., 95, 3867–72, 1998).

On the other hand, it is known that N-methyl-N′-nitro-N-nitrosoguanidine (MNNG) or H₂O₂ causes cell death by activating PARP (Halappanavar, S. S. et al., J. Biol. Chem., 274, 37097–37104, 1999; Watson, A. J., Askew, J. N. and Benson, R. S., Gastroenterology, 109, 472–482, 1995), and it is known that 3,4-dihydro-5-[4-(1-piperidinyl)butoxy]-1(2H)-isoquinolinone (DPQ), 3-aminobenzamide or nicotinamide inhibits cell death caused by PARP through the inhibition of the activation of PARP (Takahashi, K. et al., Brain Res., 829, 46–54, 1999; Watson, A. J., Askew, J. N. and Benson, R. S., Gastroenterology, 109, 472–482, 1995)).

However, the presence of a factor existing in the downstream of PARP has not been clarified yet, and it is not clear at present about the cell death-inducing mechanism of the PARP activation.

On the other hand, a human LTRPC2 (long transient receptor potential channel 2) gene was obtained in 1998 (Genomics, 54, 1, 124–131, 1998). Regarding a mouse LTRPC2 gene, there are reports stating that it was obtained (Japanese Journal of Pharmacology, Suppl. 1, 83, 2000, Molecular Cell, 9, 163–173, 2002), but its actual nucleotide sequence has not been reported. In addition, information on a rat LTRPC2 has not been reported. It has been revealed that the human LTRPC2 gene is activated by intracellular ADP ribose or NAD and functions as a calcium permeable non-selective cation channel (Sano, Y. et al., Science, 293, 1327–1330, 2001). Though it has been suggested that the controlling mechanism of LTRPC2 by ADP-ribose may have an important role in cellular functions (Nature, 411, 6837, 595–599, 2001), it is not clear about the functioning mechanism of LTRPC2 in cells and its subsequent influence upon the cells.

In addition, expression of the LTRPC2 gene in a cell prepared by cloning an immunocyte has been revealed, but its expression in actual human blood, particularly in immunocyte-containing lymphocyte, is not clear.

As has been described in the above, the fact itself that activation of PARP induces cell death is known, but the presence of a factor existing in the downstream of PARP is not clarified and its mechanism is unclear. Thus, great concern has been directed toward a method for screening a substance capable of inhibiting cell death induced by the activation of PARP, particularly a substance which is useful as a therapeutic agent and/or a preventive agent for a disease in which cell death induced by the activation of PARP is concerned (e.g., rheumatoid arthritis, neuronal death at the time of cerebral ischemia, cell death of the heart after myocardial infarction reperfusion, autoimmune destruction of β-cells of pancreatic islets of Langerhans, cell death after shock, or inflammatory reaction by immunocyte death).

DISCLOSURE OF THE INVENTION

As a result of extensive studies, the present inventors have found for the first time that activation of the LTRPC2 protein is regulated by the activation of PARP and has a function to induce cell death. In addition, it was found also for the first time that the LTRPC2 gene is expressed in human lymphocyte and that its expression level is increased in rheumatoid arthritis.

More illustratively, as shown in Example 6 which is described later, activation of LTRPC2 protein was observed when a substance known as the activator of PARP, namely MNNG or H₂O₂, was added to a cell in which the LTRPC2 protein was expressed. In addition, as shown in Example 7, when DPQ, 3-aminobenzamide or nicotinamide as a PARP inhibitor was added together with a PARP activator (e.g., MNNG or H₂O₂), activation of the LTRPC2 protein was significantly inhibited.

Further, as shown in Example 10, cell death by a PARP activator H₂O₂ was accelerated in a cell in which the LTRPC2 protein was expressed.

Also, as shown in Example 2, the LTRPC2 gene was frequently expressed in lymphocyte carrying an immune function and, as shown in Example 12, the expression level of the LTRPC2 gene was increased in a model animal of rheumatoid arthritis as one of immune diseases.

In addition, as shown in Example 17, it was found that the LTRPC2 protein accelerates cell death by streptozotocin stimulation as a type I diabetes mellitus model.

Accordingly, by selecting a substance which inhibits activation of the LTRPC2 protein, a substance capable of inhibiting cell death induced by the activation of PARP can be identified, and furthermore, a substance useful as a therapeutic drug for a disease caused by cell death, for example, a therapeutic drug for rheumatoid arthritis as an immune disease, neuronal death after cerebral ischemia or autoimmune destruction of β-cells of pancreatic islets of Langerhans in type I diabetes mellitus, can be found out. The present invention provides a screening system based on these findings.

Accordingly, the present invention relates to

(1) a method for screening a substance useful for inhibiting cell death, characterized in that it comprises

-   -   a step of allowing a substance to be tested to contact with a         cell expressing a polypeptide consisting of the amino acid         sequence represented by SEQ ID NO:2, SEQ ID NO:4 or SEQ ID NO:14         or a variation functionally equivalent thereof, under such a         condition that the aforementioned polypeptide or a variation         functionally equivalent thereof can be activated, and

a step of analyzing inhibition of the activation of the aforementioned polypeptide or a variation functionally equivalent thereof,

(2) a process for producing a pharmaceutical composition for cell death inhibition, characterized in that it comprises

a step of carrying out screening using the screening method described in (1), and

a step of preparing a pharmaceutical preparation using a substance obtained by the aforementioned screening,

(3) (i) a polypeptide which comprises the amino acid sequence represented by SEQ ID NO:4 or SEQ ID NO:14 and also exhibiting the LTRPC2 protein activity, or (ii) a polypeptide which comprises an amino acid sequence in which one or several amino acids in the amino acid sequence represented by SEQ ID NO:4 or SEQ ID NO:14 are substituted, deleted and/or inserted and also exhibiting the LTRPC2 protein activity (with the proviso that a polypeptide consisting of the amino acid sequence represented by SEQ ID NO:2 is excluded),

(4) a polypeptide which consists of the amino acid sequence represented by SEQ ID NO:4 or SEQ ID NO:14,

(5) a polynucleotide which encodes the polypeptide according to (3) or (4),

(6) a vector which comprises the polynucleotide according to (5),

(7) a cell which comprises the vector according to (6), and

(8) a process for producing the polypeptide according to (3) or (4), characterized in that it comprises a step of culturing the cell according to (7).

BEST MODE FOR CARRYING OUT THE INVENTION

The following describes the present invention in detail.

Both of apoptosis and necrosis are included in the term “cell death” as used herein. Apoptosis is a programmed cell death which is a cell death positively induced by the cell itself under a physiological condition. On the other hand, necrosis indicates death of a part of the living body and is a cell death induced by force under a non-physiological condition.

[1] Polypeptide and Polynucleotide of the Present Invention

The polypeptides consisting of the amino acid sequences represented by SEQ ID NO:4 and SEQ ID NO:14 are natural type LTRPC2 proteins respectively derived from rat and mouse. The “polypeptide consisting of the amino acid sequence represented by SEQ ID NO:4 or SEQ ID NO:14” or a variation functionally equivalent thereof (with the proviso that a polypeptide consisting of the amino acid sequence represented by SEQ ID NO:2 is excluded) is in itself a novel polypeptide and included in the polypeptide of the present invention.

As the variation functionally equivalent of the “polypeptide consisting of the amino acid sequence represented by SEQ ID NO:4”, for example,

-   (1) a polypeptide which comprises the amino acid sequence     represented by SEQ ID NO:4 and also shows the LTRPC2 protein     activity; -   (2) a polypeptide which comprises an amino acid sequence in which     one or two or more, preferably from 1 to 5, more preferably from 1     to 3, further preferably one, of amino acids in the amino acid     sequence represented by SEQ ID NO:4 are substituted, deleted and/or     inserted and also shows the LTRPC2 protein activity; -   (3) a partial fragment of a polypeptide which consists of the amino     acid sequence represented by SEQ ID NO:4 and is also a partial     fragment showing the LTRPC2 protein activity; or -   (4) a polypeptide which comprises an amino acid sequence having a     homology of 84% or more (preferably 90% or more, more preferably 95%     or more, further preferably 98% or more, and particularly preferably     99% or more) with the amino acid sequence represented by SEQ ID NO:4     and also shows the LTRPC2 protein activity, and the like may be     exemplified.

Also, as the variation functionally equivalent of the “polypeptide consisting of the amino acid sequence represented by SEQ ID NO:14”, for example,

-   (1) a polypeptide which comprises the amino acid sequence     represented by SEQ ID NO:14 and also shows the LTRPC2 protein     activity; -   (2) a polypeptide which comprises an amino acid sequence in which     one or two or more, preferably from 1 to 5, more preferably from 1     to 3, further preferably one, of amino acids in the amino acid     sequence represented by SEQ ID NO:14 are substituted, deleted and/or     inserted and also shows the LTRPC2 protein activity; -   (3) a partial fragment of a polypeptide which consists of the amino     acid sequence represented by SEQ ID NO:14 and is also a partial     fragment showing the LTRPC2 protein activity; or -   (4) a polypeptide which comprises an amino acid sequence having a     homology of 84% or more (preferably 90% or more, more preferably 95%     or more, further preferably 98% or more, and particularly preferably     99% or more) with the amino acid sequence represented by SEQ ID     NO:14 and also shows the LTRPC2 protein activity, and the like may     be exemplified.

The term “homology” as used herein means a value obtained using the b12seq program (Tatiana A. Tatusova and Thomas L. Madden, FEMS Microbiol. Lett., 174, 247–250, 1999) of a BLAST package [sgi 32 bit version, version 2.0.12; obtained from National Center for Biotechnology Information (NCBI)]. As parameters in this case, “blastp” is used as the “program name”, and “0” as the “Gap insertion Cost value”, “0” as the “Gap elongation Cost value” and “BLOSUM62” as the “Matrix”, respectively, as the pair-wise alignment parameters.

The term a polypeptide shows “LTRPC2 protein activity” as used in this specification means that it shows the same activity of the known human natural type LTRPC2 protein consisting of the amino acid sequence represented by SEQ ID NO:2, and it illustratively means an activity to cause permeation of ions as a result of the generation of a passage for ions by the aforementioned polypeptide by itself existing in the membrane, when a PARP activator is allowed to undergo the reaction for example under the co-expression of PARP.

In this specification, whether or not a polypeptide shows “LTRPC2 protein activity” is not particularly limited, but it can be verified for example by the following method (preferably the method described in Example 6). That is, a cell expressing PARP is transfected with an expression vector comprising a polynucleotide coding for the LTRPC2 protein. ⁸⁶Rb⁺ is incorporated in advance into the thus obtained cells and then a PARP activator is added thereto. Release of ⁸⁶Rb⁺ from the cells via the LTRPC2 protein formed as the result can be verified using a change in the radioactivity of ⁸⁶Rb⁺ in the extracellular moiety or inside the cells.

A polypeptide in which an appropriate marker sequence and the like are added to the N-terminus and/or C-terminus of the amino acid sequence represented by SEQ ID NO:4 or 14 (with the proviso that it must show the LTRPC2 protein activity) can be exemplified as the “polypeptide which comprises the amino acid sequence represented by SEQ ID NO:4 or 14 and also shows the LTRPC2 protein activity” as one of the variation functionally equivalents of the “polypeptide consisting of the amino acid sequence represented by SEQ ID NO:4 or 14”. As the aforementioned marker sequence, a sequence for confirming expression of a peptide, for confirming intracellular localization or for easily carrying out purification and the like can be used, and its examples include FLAG epitope, hexa-histidine tag, hemagglutinin tag, myc epitope and the like.

Origin of the variation functionally equivalent of the “polypeptide consisting of the amino acid sequence represented by SEQ ID NO:4 or 14” is not limited to human, rat or mouse. For example, the aforementioned variation functionally equivalent include not only a natural allele mutant of the polypeptide consisting of the amino acid sequence represented by SEQ ID NO:4 or 14 in human, rat or mouse (with the proviso that it shows the LTRPC2 protein activity), or a polypeptide formed by a amino acid substitution such as single nucleotide polymorphism (SNP) or the like (with the proviso that it shows the LTRPC2 protein activity), but also a naturally existing variation functionally equivalent derived from a living thing other than human, rat and mouse [e.g., a mammal other than human and rat (e.g., hamster or dog)]. Also included are polypeptides and the like which are artificially modified by genetic engineering techniques based on these natural polypeptides, particularly the amino acid sequence represented by SEQ ID NO:4 or 14.

The polynucleotides coding for the novel polypeptides of the present invention are by themselves novel ones and therefore are included in the polynucleotide of the present invention. The polynucleotide of the present invention is preferably a polynucleotide coding for a polypeptide consisting of the amino acid sequence represented by SEQ ID NO:4 or SEQ ID NO:14, more preferably a polynucleotide comprising a sequence comprising bases of from the 84th position to the 4,610th position in the nucleotide sequence represented by SEQ ID NO:3 or comprising a sequence comprising bases of from the 36th position to the 4,559th position in the nucleotide sequence represented by SEQ ID NO:13. Further preferred is a sequence comprising bases of from the 84th position to the 4,610th position in the nucleotide sequence represented by SEQ ID NO:3. In this connection, both of DNA and RNA are included in the term “polynucleotide” as used herein.

The polypeptide or polynucleotide of the present invention can be used as a tool for screening a substance for a cell death inhibition.

[2] Cell for Screening Use and Expression Vector and Cell of the Present Invention

The polypeptide consisting of the amino acid sequence represented by SEQ ID NO:2 is a known human natural type LTRPC2 protein. The polypeptides comprising the amino acid sequences represented by SEQ ID NO:2, SEQ ID NO:4 and SEQ ID NO:14 and variation functionally equivalents thereof are generally referred to as “LTRPC2 protein”.

As the variation functionally equivalent of the “polypeptide consisting of the amino acid sequence represented by SEQ ID NO:2”, for example,

-   (1) a polypeptide which comprises the amino acid sequence     represented by SEQ ID NO:2 and also shows the LTRPC2 protein     activity; -   (2) a polypeptide which comprises an amino acid sequence in which     one or two or more, preferably from 1 to 5, more preferably from 1     to 3, further preferably one, of amino acids in the amino acid     sequence represented by SEQ ID NO:2 are substituted, deleted and/or     inserted and also shows the LTRPC2 protein activity; -   (3) a partial fragment of a polypeptide which consists of the amino     acid sequence represented by SEQ ID NO:2 and is also a partial     fragment showing the LTRPC2 protein activity; or -   (4) a polypeptide which comprises an amino acid sequence having a     homology of 84% or more (preferably 90% or more, more preferably 95%     or more, further preferably 98% or more, and particularly preferably     99% or more) with the amino acid sequence represented by SEQ ID NO:2     and also shows the LTRPC2 protein activity, or the like can be     exemplified. Similar to the case of the variation functionally     equivalent of the polypeptide consisting of the amino acid sequence     represented by SEQ ID NO:4 or SEQ ID NO:14, its origin is not     limited, and a polypeptide artificially modified by genetic     engineering techniques is also included therein.

The cell to be used in the screening method of the present invention (to be called cell for screening) is not particularly limited, with the proviso that it is a cell expressing the aforementioned LTRPC2 protein, and any one of the cells produced by genetic engineering techniques and the naturally occurring cells can be used, but it is desirable to use a cell produced by genetic engineering techniques. Also, a cell expressing a polypeptide consisting of the amino acid sequence represented by SEQ ID NO:2 or a variation functionally equivalent thereof is desirable, and a cell expressing a polypeptide consisting of the amino acid sequence represented by SEQ ID NO:2, SEQ ID NO:4 or SEQ ID NO:14 is more desirable. The cell for screening can be used as a screening tool for a substance for cell death inhibition.

A vector containing the polynucleotide of the present invention and a cell containing this vector are respectively novel and included in the vector of the present invention and the cell of the present invention. The cell of the present invention is included in the aforementioned cell for screening use and can be used in the screening of the present invention.

[3] Method for Producing Cell for Screening Use

Though the method for producing cell for screening use is not particularly limited, it can be obtained, for example, by ligating a polynucleotide coding for the LTRPC2 protein into an appropriate vector DNA and transforming a host cell (preferably a eucaryote, particularly preferably CHO cell). Also, it is possible to effect expression of the polynucleotide in respective host cell by introducing an appropriate promoter and a sequence related to gene expression into such a vector.

The method for producing a polynucleotide coding for the LTRPC2 protein (includes the polynucleotide of the present invention), which can be used for the production of the cell for screening use, is not particularly limited, but (1) a method which uses PCR, (2) a method which uses a usual genetic engineering technique (namely, a method in which a transformant containing the cDNA of interest is selected from transformants transformed with a cDNA library), (3) a chemical synthesis method or the like can be exemplified. The following describes respective production methods one by one. In this connection, though the following descriptions are described with reference to the polynucleotide of the present invention, LTRPC2 protein-encoding polynucleotides other than the polynucleotide of the present invention can also be produced in the same manner.

By the method which uses PCR, the polynucleotide of the present invention can be produced for example by the following procedure.

That is, mRNA is extracted from a cell or tissue having the ability to produce the polypeptide of the present invention. Next, based on the nucleotide sequence of a polynucleotide coding for the polypeptide of the present invention, a set of two primers which can sandwich complete length mRNA corresponding to the polypeptide of the present invention or a set of two primers which can sandwich a partial mRNA region thereof are prepared. By carrying out the reverse transcriptase-polymerase chain reaction (RT-PCR), complete length cDNA coding for the polypeptide of the present invention or a part thereof can be obtained.

More illustratively, total RNA containing the mRNA coding for the polypeptide of the present invention is extracted by a known method from a cell or tissue having the ability to produce the polypeptide of the present invention. Examples of the extraction method include a guanidine thiocyanate hot phenol method, a guanidine thiocyanate-guanidine hydrochloride method, a guanidine thiocyanate cesium chloride method and the like, but it is desirable to use the guanidine thiocyanate cesium chloride method. The cell or tissue having the ability to produce the polypeptide of the present invention can be specified for example by the Northern blotting method which uses a polynucleotide coding for the polypeptide of the present invention or a part thereof or the Western blotting method which uses an antibody specific for the polypeptide of the present invention.

Subsequently, the thus extracted mRNA is purified. Purification of the mRNA can be carried out in accordance with a conventional method; for example, it can be purified by effecting adsorption of the mRNA to an oligo(dT) cellulose column and then eluting the same. As occasion demands, the mRNA can be further fractionated by a sucrose density gradient centrifugation or the like method. In addition, a commercially available already extracted and purified mRNA can also be used without carrying out extraction of the mRNA.

Next, a single-stranded cDNA is synthesized by carrying out a reverse transcriptase reaction of the thus purified mRNA in the presence, for example, of a random primer, an oligo(dT) primer and/or a custom-synthesized primer. This synthesis can be carried out by a conventional method. The cDNA of interest can be amplified by carrying out PCR using the thus obtained single-stranded cDNA and two primers sandwiching the complete length or a partial region of the polynucleotide of interest. The thus obtained DNA is fractionated by an agarose gel electrophoresis or the like. As occasion demands, a DNA fragment of interest can also be obtained by digesting the aforementioned DNA with restriction enzymes and the like and ligating the fragments. In addition, a DNA fragment of interest can also be obtained from a genomic DNA.

According to the method which uses a usual genetic engineering technique, the polynucleotide of the present invention can be produced, for example, by the procedure described in b) The second production method of 1) Method for producing protein genes in “Mode for Carrying Out the Invention” of WO 01/34785.

Regarding the method which uses a chemical synthesis, the polynucleotide of the present invention can be produced, for example, by the procedures described in c) The third production method and d) The fourth production method of 1) Method for producing protein genes in “Mode for Carrying Out the Invention” of the aforementioned patent document. More illustratively, it can also be obtained by ligating nucleotide fragments produced by a chemical synthesis method. In addition, each polynucleotide (oligonucleotide) can be synthesized using a DNA synthesizer (e.g., Oligo 1000M DNA Synthesizer (manufactured by Beckman), 394 DNA/RNA Synthesizer (manufactured by Applied Biosystems) or the like).

Sequence determination of the DNA samples obtained by the various methods so far described can be carried out, for example, by the Maxam-Gilbert chemical modification method (Maxam, A. M. and Gilbert, W., “Methods in Enzymology”, 65, 499–559, 1980), the dideoxy nucleotide chain termination method (Messing, J. and Vieira, J., Gene, 19, 269–276, 1982).

A host cell (preferably a eucaryote) can be transformed by integrating the thus isolated polynucleotide of the present invention into an appropriate vector DNA. Also, it is possible to effect expression of the polynucleotide in respective host cell by introducing an appropriate promoter and a sequence related to gene expression into such a vector.

As the host cell and expression vector which can be used for the production of the cell for screening use, for example, known host cell and expression vector generally usable in the genetic engineering can be used.

For example, cells of a vertebrate, an insect, a yeast and the like are included in the host cell of a eucaryote, and examples of the vertebrate cells include a monkey cell COS cell (Gluzman, Y., Cell, 23, 175–182, 1981), CHO dhfr⁻ cell which is a dihydrofolate reductase-deficient strain of Chinese hamster ovary cell (CHO) (Urlaub, G. and Chasin, L. A., Proc. Natl. Acad. Sci. USA, 77, 4216–4220, 1980), a human fetal kidney derived HEK 293 cell and a 293-EBNA cell (manufactured by Invitrogen) prepared by introducing Epstein-Barr virus EBNA-1 gene into the aforementioned HEK 293 cell.

When CHO cell is used as the host cell, a transformed cell which stably produce the LTRPC2 protein can be obtained by co-transfecting a vector capable of expressing a neo gene which functions as a G418 resistance marker, such as pRSVneo (Sambrook, J. et al., “Molecular Cloning—A Laboratory Manual”, Cold Spring Harbor Laboratory, NY, 1989), pSV2-neo (Southern, P. J. and Berg, P., J. Mol. Appl. Genet., 1, 327–341, 1982) or the lie, together with an expression vector containing a polynucleotide coding for the LTRPC2 protein, and by selecting a G418-resistant colony.

As the expression vector for vertebrate cells, a vector having a promoter generally positioned at the upstream of the polynucleotide to be expressed, an RNA splicing site, polyadenylation site, transcription termination sequence and the like can be used, which may further have a replication origin as occasion demands. Examples of the aforementioned expression vector include pSV2dhfr having SV40 early promoter (Subramani, S. et al., Mol. Cell. Biol., 1, 854–864, 1981), pEF-BOS having a human elongation factor promoter (Mizushima, S. and Nagata, S., Nucleic Acids Res., 18, 5322, 1990), pCEP4 having a cytomegalovirus promoter (manufactured by Invitrogen), pcDNA3.1 (+) (manufactured by Invitrogen) and the like.

The aforementioned expression vector can be incorporated into cells for example by the DEAE-dextran method (Luthman, H. and Magnusson, G., Nucleic Acids Res., 11, 1295–1308, 1983), the calcium phosphate-DNA coprecipitation method (Graham, F. L. and van der Ed, A. J., Virology, 52, 456–457, 1973), a method which uses a commercially available transfection reagent (e.g., FuGENE™6 Transfection Reagent; manufactured by Roche Diagnostics), the electroporation method (Neumann, E. et al., EMBO J., 1, 841–845, 1982) or the like.

The cell containing a polynucleotide coding for the LTRPC2 protein can be cultured in accordance with a conventional method, and the LTRPC2 protein is produced on the cell surface by the aforementioned culturing. As the medium which can be used in the aforementioned culturing, various media conventionally used in response to the employed host cells can be optionally selected. For example, in the case of CHO cell, a medium prepared by adding a serum component such as fetal bovine serum (FBS) or the like, as occasion demands to, for example, Dulbecco's modified Eagle's minimum essential medium or the like medium can be used. Also, in the case of the 293 cell, a medium prepared by adding G418 to Dulbecco's modified Eagle's minimum essential medium or the like medium supplemented with a serum component such as a fetal bovine serum (FBS) or the like can be used.

[4] Method for Screening a Substance Useful for Cell Death Inhibition

The method of the present invention for screening a substance useful for cell death inhibition includes (1) a step of allowing a substance to be tested to contact with a cell expressing the LTRPC2 protein under such a condition that the LTRPC2 protein can be activated and (2) a step of analyzing inhibition of the activation of the LTRPC2 protein.

According to the screening method of the present invention, a substance to be tested is allowed to contact with the cell for screening use under such a condition that the LTRPC2 protein can be activated. As the condition under which the LTRPC2 protein can be activated, in the presence of a LTRPC2 protein activator can for example be cited. As the aforementioned LTRPC2 protein activator, ADP ribose or NAD or a PARP activator (e.g., MNNG or H₂O₂) can for example be cited.

According to the screening method of the present invention, though not particularly limited, for example, the cell can be contacted with a substance to be tested after activation of the LTRPC2 protein or the cell can be contacted with a substance to be tested simultaneously with the activation of the LTRPC2 protein. In the former case, activation of the LTRPC2 protein is inhibited when the substance to be tested is a substance which inhibits activation of the LTRPC2 protein, and on the contrary, activation of the LTRPC2 protein is maintained when the substance to be tested is not a substance which inhibits activation of the LTRPC2 protein. In the latter case, on the other hand, activation of the LTRPC2 protein does not occur or is attenuated when the substance to be tested is a substance which inhibits activation of the LTRPC2 protein. On the contrary, activation of the LTRPC2 protein occurs when the substance to be tested is not a substance which inhibits activation of the LTRPC2 protein.

When the screening method of the present invention is used, a substance which inhibits activation of the LTRPC2 protein (namely an antagonist) can be screened. As has been found for the first time by the inventors, activation of the LTRPC2 protein is controlled by the activation of PARP. In addition, it is well known that cell death is induced by the activation of PARP. Accordingly, it has been revealed for the first time by the inventors that activation of the LTRPC2 protein is induced by the activation of PARP, and cell death is further induced as the result. Thus, a substance which inhibits activation of the LTRPC2 protein can inhibit a cell death induced by the activation of PARP and therefore is useful as the active ingredient of a therapeutic and/or preventive agent for a disease in which the cell death induced by the activation of PARP is concerned.

More illustratively, based on the difference in the method for analyzing activation of the LTRPC2 protein, for example,

(a) a screening method which uses a patch-clamp method,

(b) a screening method which uses influx or efflux of a radioisotope ion,

(c) a screening method which uses efflux of Rb⁺ ion, or

(d) a screening method which uses an intracellular Ca⁺ indicator may be exemplified as the screening method of the present invention.

When a substance which inhibits activation of the LTRPC2 protein and is useful as a cell death inhibitor is screened making use of the aforementioned patch-clamp method of (a), whether or not the activation of LTRPC2 protein is inhibited can be analyzed by measuring whole cell current in the cell for screening use using, for example, a whole-cell patch-clamp method (Hille, B., “Ionic Channels of Excitable Membranes”, 2nd Ed., 1992, Sinauer Associates Inc., Mass.). That is, though not particularly limited, the screening method of the present invention which uses a patch-clamp method includes, for example, a step in which the cell for screening use membrane potential-clamped by the whole-cell patch-clamp method and said cell for screening use is allowed to contact with a substance to be tested under such a condition that the LTRPC2 protein can be activated and a step in which a change in the whole cell current in said cell for screening use is analyzed.

More illustratively, whole cell current in the cell for screening use is measured by membrane potential-clamped the cell for screening use by the whole-cell patch-clamp method and adding an LTRPC2 protein activating compound (e.g., ADP ribose, NAD, H₂O₂ or MNNG). In that case, a solution containing 145 mmol/L NaCl, 5 mmol/L KCl, 2 mmol/L CaCl₂, 2 mmol/L MgCl₂ and 10 mmol/L HEPES-Na (pH 7.4) is used as the extracellular solution, and a solution containing 150 mmol/L CsCl, 5 mmol/L MgCl₂ and 10 mmol/L HEPES-Cs (pH 7.2) can be used as the intracellular solution. By subsequently measuring changes in the current when a substance to be tested is added to the extracellular solution or intracellular solution, a substance which inhibits activation of the LTRPC2 protein can be screened. For example, in case that change in the whole cell current which is generated by the activation stimulus of the LTRPC2 protein disappears or is attenuated in the cell for screening use when a substance to be tested is added, it can be judged that said substance to be tested is a substance which inhibits activation of the LTRPC2 protein.

When a substance which inhibits activation of the LTRPC2 protein and is useful as a cell death inhibitor is screened making use of the aforementioned efflux of radioisotope ion of (b), its channel activity can be measured for example using respective radioisotope of Ca²⁺, Na⁺ or K⁺ ion as the index (Sidney P. Colowick and Nathan O. Kaplan “Methods in Enzymology”, 88 (1), 1982, Academic Press, 346–347). This analyzing method is based on a well known finding that the LTRPC2 protein causes permeation of Ca²⁺, Na⁺ or K⁺ ion (Sano, Y. et al., Science, 293, 1327–1330, 2001).

Though not particularly limited, the screening method of the present invention which uses efflux of a radioisotope ion includes, for example, a step in which a radioisotope is incorporated into the cell for screening use and then said cell for screening use is allowed to contact with a substance to be tested under such a condition that the LTRPC2 protein can be activated and a step in which amount of the radioactivity discharged into the extracellular moiety of the cell for screening use, or amount of the radioactivity remained in the cell is analyzed.

More illustratively, it can be measured using ⁴⁵Ca²⁺, ²²Na⁺ or ⁴²K⁺ as the radioisotope of each ion. When K⁺ is used, K⁺ is incorporated into the cell in advance and then K⁺ remained in the extracellular solution is removed. Thereafter, since the radioisotope K⁺ is permeated when the LTRPC2 protein is activated using an LTRPC2 protein activator, the radioactivity in the extracellular solution (namely, the radioactivity discharged into the extracellular moiety) or the radioactivity remained in the cell can be used as the index of the channel activity. In case that ⁴⁵Ca²⁺ or ²²Na⁺ is used, the radioisotope permeates when the LTRPC2 protein is activated using an LTRPC2 protein activator under a condition of putting the ⁴⁵Ca²⁺ or ²²Na⁺ in the reaction solution, so that the radioactivity in the extracellular solution (namely, the radioactivity remained in the extracellular moiety) or the radioactivity influxed into the cell can be used as the index of the channel activity (Experimental Medicine Supplement “Cultured Cell Experimentation Methods for Molecular Biology Studies”, edited by T. Kuroki, N. Kyo and K. Senda, 1995, published by Yodosha; to be referred to as reference 1 hereinafter). In that case, a substance which inhibits activation of the LTRPC2 protein can be screened by adding a substance to be tested in the presence of an LTRPC2 protein activator and measuring the radioactivity in the extracellular solution (namely, the radioactivity discharged into the extracellular moiety) or the radioactivity remained in the cell. More illustratively, it can be carried out in accordance with the “(c) screening method which uses efflux of Rb⁺ ion” which is described below.

In general, an ion channel capable of effecting permeation of K⁺ ion can also pass Rb⁺ ion similar to the case of K⁺ ion, so that the channel activity can be measured using efflux of a radioisotope ⁸⁶Rb⁺ as the index (Sidney P. Colowick and Nathan O. Kaplan “Methods in Enzymology”, 88 (1), 1982, Academic Press, 346–347; to be referred to as reference 2 hereinafter).

Though not particularly limited, the screening method of the present invention in which a substance which inhibits activation of the LTRPC2 protein and is useful as a cell death inhibitor is screened making use of the aforementioned efflux of Rb⁺ ion of (c) includes, for example, a step in which a radioisotope ⁸⁶Rb⁺ ion is incorporated into the cell for screening use and then said cell for screening use is allowed to contact with a substance to be tested under such a condition that the LTRPC2 protein can be activated and a step in which amount of the radioactivity of ⁸⁶Rb⁺ discharged into the extracellular moiety of the cell for screening use, or amount of the radioactivity of ⁸⁶Rb⁺ remained in the cell is analyzed.

According to the screening method of the present invention which uses efflux of Rb⁺ ion, ⁸⁶Rb⁺ can be incorporated into the aforementioned cell by incubating the cell for screening use together with ⁸⁶RbCl (e.g., at 37° C. for 24 hours) (reference 1), The un-incorporated ⁸⁶Rb⁺ is removed by washing the cell with usual physiological saline. Since the amount of ⁸⁶Rb⁺ discharged into the extracellular moiety is increased when the LTRPC2 protein is activated, the radioactivity of ⁸⁶Rb⁺ in the extracellular solution or the radioactivity of ⁸⁶Rb⁺ remained in the cell can be used as the index of the channel activity.

More illustratively, for example, the cell for screening use incorporated in advance with ⁸⁶Rb⁺ is incubated (e.g., at 37° C. for 30 minutes) in a solution (e.g., physiological saline) containing a substance to be tested, in the presence of an LTRPC2 protein, and then the radioactivity of ⁸⁶Rb⁺ in the extracellular solution or the radioactivity of ⁸⁶Rb⁺ remained in the cell is measured. In that case, it is desirable to carry out the same operation as a negative control using a solution which does not contain the substance to be tested and in the absence of the LTRPC2 protein activator, and to carry out the same operation as a positive control using a solution which does not contain the substance to be tested and in the presence of the LTRPC2 protein activator. Since the LTRPC2 protein is not activated in the aforementioned negative control, ⁸⁶Rb⁺ is not discharged into the extracellular moiety, while ⁸⁶Rb⁺ is discharged into the extracellular moiety in the positive control because of the activation of LTRPC2 protein. When (1) discharge of ⁸⁶Rb⁺ into the extracellular moiety is nor observed similar to the case of the aforementioned negative control or (2) discharged amount of ⁸⁶Rb⁺ into the extracellular moiety is reduced in comparison with the aforementioned positive control, in case that a substance to be tested is added in the presence of an LTRPC2 protein activator, it can be judged that the aforementioned substance to be tested is a substance which inhibits activation of the LTRPC2 protein. It is desirable that the screening method of the present invention which uses discharge of Rb⁺ ion is carried out under the conditions which are described later in Example 7.

When a substance which inhibits activation of the LTRPC2 protein and is useful as a cell death inhibitor is screened making use of the aforementioned intracellular Ca⁺ indicator of (d), Fluo3-AM or the like can for example be used as the intracellular Ca⁺ indicator. An intracellular Ca⁺ indicator can optically detect changes in the intracellular Ca⁺ concentration accompanied by the opening of the LTRPC2 protein (Experimental Medicine Supplement “Intracellular Calcium Experimentation Protocol”, edited by S. Kudo, 1996, published by Yodosha). Activity of the LTRPC2 protein can be measured by the use of these indicators, and it is possible to screen a compound which inhibits activation of the LTRPC2 protein, by comparing changes in the amount at the time of the activation of LTRPC2 protein in the presence and absence of a substance to be tested. Though not particularly limited, the screening method of the present invention which uses an intracellular Ca⁺ indicator includes, for example, a step in which an intracellular Ca⁺ indicator is incorporated into the cell for screening use and then said cell for screening use is allowed to contact with a substance to be tested under such a condition that the LTRPC2 protein can be activated and a step in which changed amount of the intracellular Ca⁺ indicator in the aforementioned cell for screening use is optically analyzed.

More illustratively, when the amount of Ca⁺ permeating into the cell is reduced or disappears in comparison with the case of the absence of a substance to be tested, in case that the substance to be tested is added in the presence of an LTRPC2 protein activator, it can be judged that the aforementioned substance to be tested is a substance which inhibits activation of the LTRPC2 protein.

The substances to be tested as the objects to be selected by the screening method of the present invention are not particularly limited, but various conventionally known compounds (including peptides) registered in Chemical File, a group of compounds obtained by the combinatorial chemistry techniques (Terrett, N. K. et al., Tetrahedron, 51, 8135–8137, 1995), or a group of random peptides prepared by applying the phage display method (Felici, F. et al., J. Mol. Biol., 222, 301–310, 1991) and the like can be used. Also, microorganisms, plants, marine organisms, animal-derived natural components (e.g., culture supernatants or tissue extracts) and the like can also be used as the substances to be tested by the screening. In addition, compounds (including peptides) prepared by chemically or biologically modifying the compounds (including peptides) selected by the screening method of the present invention can also be used.

[5] Method for Producing Pharmaceutical Composition for Cell Death Inhibition

A method for producing a pharmaceutical composition for cell death inhibition, characterized in that it comprises a step of carrying out a screening using the screening method of the present invention and a step of preparing a pharmaceutical preparation using a substance obtained by the aforementioned screening, is included in the present invention.

The pharmaceutical preparation which comprises a substance obtained by the screening method of the present invention as the active ingredient can be prepared by using carriers, fillers and/or other additive agents generally used in preparing medicaments, in response to the type of the aforementioned active ingredient.

Examples of the administration include oral administration by tablets, pills, capsules, granules, fine subtilaes, powders, solutions for oral use and the like and parenteral administration by injections (e.g., intravenous, intramuscular, intraarticular or the like), suppositories, percutaneous preparations, transmucosal preparations and the like. Particularly in the case of peptides which are digested in the stomach, parenteral administration such as intravenous injection or the like is desirable.

In the solid composition for oral administration, one or more active substances can be mixed with at least one inert diluent such as lactose, mannitol, glucose, microcrystalline cellulose, hydroxypropylcellulose, starch, polyvinyl pyrrolidone, aluminum magnesium silicate or the like. In the usual way, the aforementioned composition may contain other additives than the inert diluent, such as a lubricant, a disintegrating agent, a stabilizing agent, a solubilizing or solubilization assisting agent and the like. If necessary, tablets or pills may be coated with a sugar coating or a film of a gastric or enteric substance.

The liquid composition for oral administration can include, for example, emulsions, solutions, suspensions, syrups or elixirs and can contain a generally used inert diluent such as purified water or ethanol. In addition to the inert diluent, the aforementioned composition can also contain a moistening agent, a suspending agent a sweetener, an aromatic or an antiseptic.

The injections for parenteral administration can include aseptic aqueous or non-aqueous solutions, suspensions or emulsions. As a diluent, the water-soluble aqueous solutions or suspensions can include distilled water for injection, physiological saline or the like. Examples of the diluent for use in the non-aqueous solutions or suspensions include propylene glycol, polyethylene glycol, plant oil (e.g., olive oil), alcohols (e.g., ethanol), polysorbate 80 or the like. The aforementioned composition can further contain a moistening agent, an emulsifying agent, a dispersing agent, a stabilizing agent, a solubilizing or solubilization assisting agent, an antiseptic or the like. The aforementioned composition can be sterilized, for example, by filtration through a bacteria retaining filter, blending of a germicide or irradiation. Alternatively, it can be used by firstly making into a sterile solid composition and dissolving it in sterile water or a sterile solvent for injection prior to its use.

The dose can be optionally decided by taking into consideration strength of the activity of the active ingredient, namely a substance which inhibits activation of the LTRPC2 protein or a substance obtained by the screening method of the present invention, and symptoms, age, sex and the like of each patient to be treated.

For example, in the case of oral administration, the dose is usually from 0.1 to 100 mg, preferably from 0.1 to 50 mg, per day per adult (as 60 kg body weight). In the case of parenteral administration, it is from 0.01 to 50 mg, preferably from 0.01 to 10 mg, as a mode of injections.

EXAMPLES

The following illustratively describes the present invention based on examples, but they do not limit the scope of the present invention. In this connection, unless otherwise noted, genetic engineering techniques and channel activity analysis can be carried out in accordance with the conventionally known methods (Maniatis, T. et al., “Molecular Cloning—A Laboratory Manual”, 1982, Cold Spring Harbor Laboratory, NY; Hille, B., “Ionic Channels of Excitable Membranes”, 2nd Ed., 1992, Sinauer Associates Inc., MA).

In carrying out cloning in the examples, SUPERSCRIPT First-Strand Synthesis System for RT-PCR manufactured by Invitrogen was used as the kit for reverse transcription reaction, and TOPO XL PCR Cloning Kit manufactured by Invitrogen as the cloning kit.

Example 1

<Isolation of Gene Coding for Human LTRPC2 Protein and Construction of Expression Vector>

A complete length cDNA coding for a human LTRPC2 protein consisting of the amino acid sequence represented by SEQ ID NO:2 (to be referred simply to as “human LTRPC2 protein” in each of the following Examples) was obtained by the reverse transcription-polymerase chain reaction (RT-PCR) using a human leukocyte mRNA as the temperate. Firstly, a single-stranded cDNA was synthesized by carrying out reverse transcription of the human leukocyte mRNA (10 ng) using the kit for reverse transcription reaction. Using this single-stranded cDNA as the template, PCR was carried out by a hot start method using Taq DNA polymerase (LA Taq DNA polymerase; manufactured by Takara Shuzo). The aforementioned PCR was carried out using an oligonucleotide comprising the nucleotide sequence represented by SEQ ID NO:5 as the sense primer, and an oligonucleotide comprising the nucleotide sequence represented by SEQ ID NO:6 as the antisense primer, and firstly carrying out thermal denaturation at 98° C. (1 minute) and then repeating a cycle of 98° C. (15 seconds)/65° C. (30 seconds)/72° C. (6 minutes) 35 times. As a result, a DNA fragment of about 4.7 kbp was amplified.

This DNA fragment was cloned into pCR-TOPO vector using the cloning kit. LTRPC2-cDNA alone was isolated by digesting the thus obtained plasmid DNA with a restriction enzyme EcoRI and then cloned using a pcDNA3.1 (+) plasmid (manufactured by Invitrogen). Since the pcDNA3.1 (+) plasmid has a cytomegalovirus-derived promoter sequence, it can be used for expressing the LTRPC2 protein in animal cells. When nucleotide sequence of the thus obtained clone pcDNA3.1-LTRPC2 was analyzed by the dideoxy terminator method using a DNA sequencer (ABI 3700 DNA Sequencer; manufactured by Applied Biosystems), the sequence represented by SEQ ID NO:1 was obtained.

Example 2

<Expression Distribution of Human LTRPC2 Gene>

Expression distribution of human LTRPC2 gene in human tissues and human blood was analyzed by the RT-PCR method. Regarding the human tissues, 5 ng of each of mRNA preparations derive from respective human tissues (manufactured by Clontech) was subjected to a DNase treatment and then to reverse transcription using a kit for reverse transcription reaction (Advantage RT-for-PCR Kit; manufactured by Clontech), thereby obtaining a single-stranded cDNA. Using this single-stranded cDNA as the template, PCR was carried out by a hot start method using Taq DNA polymerase (LA Taq DNA polymerase; manufactured by Takara Shuzo).

A 100 ml portion of human blood was collected from an anonymous in-house volunteer (healthy person), using a syringe containing heparin sodium for preventing blood coagulation. The blood sample was fractionated into eosinophil, neutrophil, lymphocyte and platelet using Ficoll-Paque reagent (manufactured by Amersham Pharmacia), and then total RNA was extracted from each of them using ISOGEN reagent (manufactured by Nippon Gene). In this case, the actual operation was carried out in accordance with respective protocols attached to the Ficoll-Paque reagent and ISOGEN reagent. A single-stranded cDNA was obtained by subjecting 20 ng of each total RNA to a DNase treatment and then to reverse transcription using the kit for reverse transcription reaction. Using this single-stranded cDNA as the template, PCR was carried out by a hot start method using Taq DNA polymerase (LA Taq DNA polymerase; manufactured by Takara Shuzo).

The aforementioned PCR was carried out using an oligonucleotide comprising the nucleotide sequence represented by SEQ ID NO:7 as the sense primer, and an oligonucleotide comprising the nucleotide sequence represented by SEQ ID NO:8 as the antisense primer, and firstly carrying out thermal denaturation at 98° C. (1 minute) and then repeating a cycle of 98° C. (15 seconds)/70° C. (1 minute) 30 times. Each of these primers is a sequence specific for the human LTRPC2 gene, comprising a sequence comprising bases of from the 22nd to the 4,533rd positions in the nucleotide sequence represented by SEQ ID NO:1.

When RT-PCR analysis was carried out on respective human tissues (amygdala, caudate nucleus, hippocampus, corpus callosum, substantia nigra, thalamus, cerebellum, frontal lobe, hypothalamus, spinal cord, pituitary body, whole brain, heart, placenta, lung, trachea, liver, skeletal muscle, kidney, small intestine, stomach, spleen, bone marrow, thymus, thyroid gland, salivary gland, adrenal gland, mammary gland and prostate), a DNA fragment of about 660 bp was amplified in brain, spinal cord, heart, placenta, lung, trachea, small intestine, stomach, spleen, bone marrow, thymus and leukocyte. It was revealed based on this result that the mRNA of human LTRPC2 is expressed in tissues which have been reported to be related to diseases whose cause is a cell death accompanied by the PARP activation.

In addition, when the RT-PCR analysis was carried out on human blood, a DNA fragment of about 660 bp was amplified mainly in lymphocyte. It was revealed based on this result that the human LTRPC2 is expressed mainly in lymphocyte in the human blood. Since it has been revealed that lymphocyte carries an immune function, it was suggested that the LTRPC2 gene is taking an important role in an immune function.

Example 3

<Isolation of Gene Coding for Rat LTRPC2 Protein>

A complete length cDNA coding for a rat LTRPC2 protein consisting of the amino acid sequence represented by SEQ ID NO:4 (to be referred simply to as “rat LTRPC2 protein” in each of the following Examples) was obtained by RT-PCR using 10 ng of a rat brain mRNA (manufactured by Clontech) as the temperate, by the same techniques of Example 1. PCR was carried out using an oligonucleotide comprising the nucleotide sequence represented by SEQ ID NO:9 as the sense primer, and an oligonucleotide comprising the nucleotide sequence represented by SEQ ID NO:10 as the antisense primer, and firstly carrying out thermal denaturation at 98° C. (1 minute) and then repeating a cycle of 98° C. (15 seconds)/60° C. (30 seconds)/72° C. (5 minutes) 35 times. As a result, a DNA fragment of about 4.7 kbp was amplified. When this DNA fragment was cloned into pCR-TOPO vector and nucleotide sequence of the thus obtained clone pCR-TOPO-rat LTRPC2 was analyzed, the sequence represented by SEQ ID NO:3 was obtained. The nucleotide sequence represented by SEQ ID NO:3 has an open reading frame comprising 4,527 base pairs (a sequence comprising bases of from the 84th to 4,610th positions in the nucleotide sequence represented by SEQ ID NO:3). Amino acid sequence deduced from the aforementioned open reading frame was the amino acid sequence represented by SEQ ID NO:4 comprising 1,508 amino acid residues.

The amino acid sequence represented by SEQ ID NO:4 has a homology of 84% with the amino acid sequence represented by SEQ ID NO:2 (human LTRPC2 protein). In this connection, the aforementioned numerical value of homology is a value obtained by the aforementioned BLAST retrieval.

Example 4

<Expression Distribution of Rat LTRPC2 Gene>

Expression distribution of rat LTRPC2 gene in rat tissues was analyzed by the RT-PCR method. A 5 ng portion of each of mRNA preparations derive from respective rat tissues (manufactured by Clontech) was subjected to a DNase treatment and then to reverse transcription using the kit for reverse transcription reaction, thereby obtaining a single-stranded cDNA. Using this single-stranded cDNA as the template, PCR was carried out by a hot start method using a Taq DNA polymerase (Platinum Taq DNA polymerase; manufactured by Invitrogen). The aforementioned PCR was carried out using an oligonucleotide comprising the nucleotide sequence represented by SEQ ID NO:11 as the sense primer, and an oligonucleotide comprising the nucleotide sequence represented by SEQ ID NO:12 as the antisense primer, and firstly carrying out thermal denaturation at 95° C. (1 minute) and then repeating a cycle of 95° C. (15 seconds)/62° C. (30 seconds)/72° C. (1 minute) 35 times. Each of these primers is a sequence specific for the rat LTRPC2 gene, comprising a sequence comprising bases of from the 84th to 4,610th positions in the nucleotide sequence represented by SEQ ID NO:3.

When RT-PCR analysis was carried out on respective rat tissues (brain, heart, kidney, liver, lung, pancreas, retina, skeletal muscle, spleen and testis), a DNA fragment of about 380 bp was amplified in all of the tissues, thus revealing that the mRNA of rat LTRPC2 is expressed in each of the aforementioned tissues. In addition, it was revealed based on this result that the mRNA of rat LTRPC2 is expressed in tissues which have been reported to be related to diseases whose cause is a cell death accompanied by the PARP activation.

Example 5

<Expression of Human LTRPC2 Protein in Animal Cell>

In order to detect channel activity of the human LTRPC2 protein, expression of the aforementioned protein was induced in an animal cell. Using the expression vector pcDNA3.1-LTRPC2 obtained in Example 1 and a reagent for transfection (LIPOFECTAMINE 2000; manufactured by Invitrogen), transfection of a Chinese hamster ovary-derived CHO dhfr⁻ cell was carried out to induce expression of the human LTRPC2 protein. In this case, the aforementioned operation was carried out in accordance with the protocol attached to the aforementioned reagent for transfection and the known method (reference 1).

Example 6

<Detection of Activation of Human LTRPC2 Protein by PARP Activator>

This was carried out in accordance with the known methods (reference 1 and reference 2), except for the operation illustratively described in the following.

The transfected cells obtained in Example 5 (1.6×10⁵ cells) were incubated at 37° C. for 24 hours in the presence of ⁸⁶RbCl (1 μCi/ml) to effect incorporation of ⁸⁶Rb⁺ into the cells and then washed with physiological saline to remove the ⁸⁶Rb⁺ which was not incorporated into the cells. The thus obtained cells were incubated at room temperature for 30 minutes in physiological saline supplemented with a PARP activator MNNG (final concentration=1 mmol/liter) or H₂O₂ (final concentration=0.06%). In this connection, the same operation was carried out as a control in general physiological saline (namely, physiological saline to which MNNG or H₂O₂ was not added).

The respective cells were washed with physiological saline, and then radioactivity of the ⁸⁶Rb⁺ remained in the cells was measured. As a result, residual activity of ⁸⁶Rb⁺ was reduced to 35.6% in the cells to which MNNG was added, and to 8.9% in the cells to which H₂O₂ was added, respectively, in comparison with the control cells. (cells to which MNNG or H₂O₂ was not added). This shows that ⁸⁶Rb⁺ in the cells was effluxed into the extracellular moiety caused by the activation of human LTRPC2 protein induced by MNNG or H₂O₂. It was revealed based on this result that the human LTRPC2 protein is activated by a PARP activator MNNG or H₂O₂.

Example 7

<Inhibition of Activation of Human LTRPC2 Protein by PARP Inhibitor>

This was carried out in accordance with the known methods (reference 1 and reference 2), except for the operation illustratively described in the following.

The transfected cells obtained in Example 5 (1.6×10⁵ cells) were incubated at 37° C. for 24 hours in the presence of ⁸⁶RbCl (1 μCi/ml) to effect incorporation of ⁸⁶Rb⁺ into the cells and then washed with physiological saline to remove the ⁸⁶Rb⁺ which was not incorporated into the cells. The thus obtained cells were incubated at room temperature for 30 minutes in physiological saline supplemented with a PARP inhibitor DPQ (final concentration=100 μmol/liter), 3-aminobenzamide (final concentration=1 mmol/liter) or nicotinamide (final concentration=1 mmol/liter), in the presence of MNNG (final concentration=1 mmol/liter) or H₂O₂ (final concentration=0.06%). In this connection, the same operation was carried out as a control in physiological saline (namely, physiological saline to which DPQ, 3-aminobenzamide or nicotinamide was not added), in the absence of MNNG and H₂O₂.

The respective cells were washed with physiological saline, and then radioactivity of the ⁸⁶Rb⁺ remained in the cells was measured. As a result, residual activity of ⁸⁶Rb⁺ in respective cells to which the PARP inhibitor was added was similar to that of the control cells (cells to which MNNG or H₂O₂ was not added), namely the cells in which activity of the human LTRPC2 protein was not induced. More illustratively, the cells to which DPQ was added showed 93.4% (in the case of the presence of H₂O₂) or 95.5% (in the case of the presence of MNNG) of the ⁸⁶Rb⁺ residual activity. In the same manner, the cells to which 3-aminobenzamide was added showed 91.9% (in the case of the presence of H₂O₂) or 96.2% (in the case of the presence of MNNG) of the ⁸⁶Rb⁺ residual activity, and the cells to which nicotinamide was added showed 57.8% (in the case of the presence of H₂O₂) or 95.9% (in the case of the presence of MNNG) of the ⁸⁶Rb⁺ residual activity.

This shows that ⁸⁶Rb⁺ in the cells was not effluxed into the extracellular moiety, or the efflux was reduced, due to inhibition of the activation of human LTRPC2 protein caused by DPQ, 3-aminobenzamide or nicotinamide. It was revealed based on this result that the activation of human LTRPC2 protein by MNNG or H₂O₂ is inhibited by a PARP inhibitor DPQ, 3-aminobenzamide or nicotinamide.

Example 8

<Comparison of Human LTRPC2 Protein Inhibition and PARP Inhibition by PARP Inhibitor>

In order to verify that the human LTRPC2 protein inhibition by PARP inhibitor, shown in Example 7, occurs via PARP, IC₅₀ value of the human LTRPC2 protein inhibition and IC₅₀ value of the PARP inhibition were compared and examined.

IC₅₀ value of the LTRPC2 inhibition by each of the aforementioned PARP inhibitors DPQ, 3-aminobenzamide and nicotinamide was examined by the method of Example 7, by adding varied amounts of the inhibitors. As a result, IC₅₀ values of the human LTRPC2 protein inhibition by DPQ, 3-aminobenzamide and nicotinamide were 0.20 μM, 117 μM and 541 μM, respectively.

IC₅₀ value of the PARP inhibition was examined by the following method. The transfected cells obtained in Example 5 were incubated at room temperature for 30 minutes in physiological saline supplemented with a PARP activator MNNG (final concentration=1 mmol/liter) or H₂O₂ (final concentration=0.06%) and varied amount of the PARP inhibitor used in the above. In this connection, the same operation was carried out as a control in general physiological saline (namely, physiological saline to which MNNG or H₂O₂ was not added). Next, the cells after removal of the physiological saline supplemented with respective activator and inhibitor were incubated with 0.2 nCi/μl in final concentration of ³H-NAD at 37° C. for 40 minutes. Subsequently, the reaction was stopped by adding TCA to the cells to a final concentration of 10% and the cells were incubated at 4° C. for 30 minutes. Thereafter, the un-incorporated ³H-NAD was removed by washing the cells twice with 5% TCA. The thus washed cells were lysed with a solution containing 2% SDS and 0.1 M NaOH, and then the ³H radioactivity contained in the cells was measured. As a result, IC₅₀ values of the PARP inhibition by DPQ, 3-aminobenzamide and nicotinamide were 0.26 μM, 71.7 μM and 335 μM, respectively.

Since the respective IC₅₀ values of LTRPC2 inhibition and PARP inhibition approximated to each other, it was confirmed that the LTRPC2 inhibition was due to the PARP inhibition.

Example 9

<Expression of Human LTRPC2 Protein in Animal Cell>

In order to clarify function of the human LTRPC2 protein in cells, expression of the aforementioned protein was induced in an animal cell. Using the expression vector pcDNA3.1-LTRPC2 obtained in Example 1 and a reagent for transfection (LIPOFECTAMINE 2000; manufactured by Invitrogen), transfection of a human fetal kidney-derived HEK 293 cell was carried out to induce expression of the human LTRPC2 protein. In this case, the aforementioned operation was carried out in accordance with the protocol attached to the aforementioned reagent for transfection and the known method (reference 1).

Example 10

<Induction of Cell Death by Human LTRPC2 Protein>

The transfected cells obtained in Example 8 were inoculated in 4×10⁴ cells/well portions into a 96 well plate (manufactured by Asahi TechnoGlass), cultured overnight and then incubated at room temperature for 2 hours using a cell culture medium containing 0.001% in final concentration of H₂O₂. After washing with physiological saline, they were cultured for 4 hours by adding the cell culture medium. After removing the medium, alamaBlue Assay (provided by Bioscience) was carried out in order to judge life or death of the cells. According to this operation, life or death of the cells can be judged by adding a reagent which emits fluorescence by an oxidation reduction reaction in the cells and measuring the resulting fluorescence intensity. In this connection, the experimental procedure was carried out in accordance with the protocol attached to the reagent for the aforementioned alamaBlue Assay. The reagent for alamaBlue Assay was added to the stimulated cells, the mixture was incubated for additional 1 hour, and then the fluorescence intensity was measured. In this connection, the same operation was carried out as a control using cells transfected with pcDNA3.1 vector alone which does not contain LTRPC2. As a result, the fluorescence intensity was significantly reduced (reduction by 47.5%) in the cells in which LTRPC2 was expressed, in comparison with the cells in which the vector alone was expressed. This shows that a cell death occurred via LTRPC2 by the H₂O₂ stimulation, and surviving cells were reduced as a result. It was able to confirm based on this result that the human LTRPC2 protein accelerates the cell death by H₂O₂ stimulation. In addition, since it has been revealed by the aforementioned result that LTRPC2 is activated by the H₂O₂ stimulation, it is evident also from the result of this Example that the LTRPC2 protein has a function to induce cell death.

Example 11

<Isolation of Gene Coding for Mouse LTRPC2 Protein>

A complete length cDNA coding for a mouse LTRPC2 protein consisting of the amino acid sequence represented by SEQ ID NO:14 (to be referred simply to as “mouse LTRPC2 protein” in each of the following examples) was obtained by RT-PCR using 10 ng of a mouse brain mRNA (manufactured by Clontech) as the temperate, by the same techniques of Example 1 and Example 2. The PCR was carried out using an oligonucleotide comprising the nucleotide sequence represented by SEQ ID NO:15 as the sense primer, and an oligonucleotide comprising the nucleotide sequence represented by SEQ ID NO:16 as the antisense primer, and the PCR cycle was carried out under the same condition of Example 2. As a result, a DNA fragment of about 4.7 kbp was amplified. When this DNA fragment was cloned into the pCR-TOPO vector and nucleotide sequence of the thus obtained clone pCR-TOPO-mouse LTRPC2 was analyzed, the sequence represented by SEQ ID NO:13 was obtained. The nucleotide sequence represented by SEQ ID NO:13 has an open reading frame comprising 4,524 base pairs (a sequence comprising bases of from the 36th to 4,559th positions in the nucleotide sequence represented by SEQ ID NO:13). Amino acid sequence deduced from the aforementioned open reading frame was the amino acid sequence represented by SEQ ID NO:14 comprising 1,507 amino acid residues. The amino acid sequence represented by SEQ ID NO:14 has a homology of 84% with the amino acid sequence represented by SEQ ID NO:2 (human LTRPC2 protein) and that of 94% with the amino acid sequence represented by SEQ ID NO:4 (rat LTRPC2 protein). In this connection, the aforementioned numerical values of homology are values obtained by the aforementioned BLAST program.

Example 12

<Expression Analysis of Mouse LTRPC2 Gene in Rheumatoid Arthritis Model Animal>

In order to carry out expression analysis of LTRPC2 gene in rheumatoid arthritis, a mouse collagen-induced arthritis model was prepared as a model of rheumatoid arthritis, and changes in the expression of mouse LTRPC2 gene in this mouse were analyzed.

Preparation of the mouse collagen-induced arthritis model was carried out by the following method. A 150 μg portion of type 2 collagen was injected into a mouse joint region, the animal was fed for 3 weeks, and then injection of the same amount of collagen was carried out.

Subsequently, 5 weeks of feeding was carried out and then the joint region was excised by carrying out dissection. In this case, it was confirmed that damages (chondroclasis and osteoclasis) similar to the case of rheumatoid arthritis patients were formed in the excised mouse joint. This mouse joint was frozen and pulverized, and then mRNA was extracted using ISOGEN reagent (manufactured by Nippon Gene). In this connection, mRNA was also extracted as a control from a joint of a normal mouse without the treatment.

In order to analyze changes in the expression of mouse LTRPC2 gene in the thus obtained joint of mouse collagen-induced arthritis model, real time PCR was carried out using PRISM 7900 (manufactured by Applied Biosystems). By carrying out real time PCR, mouse LTRPC2 gene contained in mRNA can be quantified and measured.

The thus obtained mRNA was subjected to a DNase treatment and then to reverse transcription using the kit for reverse transcription reaction, thereby obtaining a single-stranded cDNA. Using this single-stranded cDNA as the template, PCR was carried out using a fluorescence reagent SYBR Green PCR Core Reagents Kit (manufactured by Applied Biosystems). The aforementioned PCR was carried out using an oligonucleotide comprising the nucleotide sequence represented by SEQ ID NO:17 as the sense primer, and an oligonucleotide comprising the nucleotide sequence represented by SEQ ID NO:18 as the antisense primer, and firstly carrying out thermal denaturation at 95° C. (10 minutes) and then repeating a cycle of 95° C. (15 seconds)/59° C. (1 minute) 45 times. These primers are sequences specific for the mouse LTRPC2 gene, comprising respective sequences of from the 3,763rd to 3,780th positions and from the 3,813rd to 3,830th positions in the nucleotide sequence represented by SEQ ID NO:13.

As a result, expressed amount of the mouse LTRPC2 gene was increased in the mouse collagen-induced arthritis model by a factor of about 15 times in comparison with the untreated normal mouse. It was revealed from this result that the mouse LTRPC2 gene is taking an important role in rheumatoid arthritis.

Example 13

<Expression Distribution of Mouse LTRPC2 Gene>

Expression distribution of mouse LTRPC2 gene in mouse tissues was analyzed by real time PCR by the same method of Example 12 using PRISM 7900 (manufactured by Applied Biosystems). A 5 ng portion of each of mRNA preparations (manufactured by Clontech) derive from respective mouse tissues (brain, heart, kidney, liver, lung, pancreas, skeletal muscle, smooth muscle, spleen and testis) was subjected to a DNase treatment and then to reverse transcription using the kit for reverse transcription reaction, thereby obtaining a single-stranded cDNA. Using this single-stranded cDNA as the template, PCR was carried out. As a result, amplification was detected in all of the tissues with the brain and spleen as the center. It was revealed from this that the mRNA of mouse LTRPC2 is expressed in each of the aforementioned tissues. In addition, it was revealed based on this result that the mRNA of mouse LTRPC2 is expressed in tissues which have been reported to be related to diseases whose cause is a cell death accompanied by the PARP activation.

Example 14

<Isolation of Gene Coding for PARP-1 Dominant Negative Mutant and Construction of Expression Vector>

A sequence of from the 1st to 374th positions of a human PARP-1 complete length amino acid sequence (Genebank accession No: XP-037273) is a DNA binding region, and it is known that when this DNA binding region alone is co-expressed wild type PARP-1, they compete for the binding to DNA. It is known that the binding ratio of wild type PARP-1 to DNA is reduced as the result, thus causing inhibition of the poly(ADP-ribosyl)ation of activated PARP-1 to DNA and inhibition of the induction of cell death (dominant negative effect). Thus, it has been revealed that the DNA binding region alone of PARP-1 functions as a dominant negative mutant protein which shows the dominant negative effect for PARP-1 (Kupper, J. H., de, Murcia, G. and Burkle, A., J. Biol. Chem., 265, 18721–18724, 1990).

A complete length cDNA coding for a human PARP-1 dominant negative mutant protein comprising the aforementioned region, namely the amino acid sequence represented by SEQ ID NO:20 (to be referred simply to as “PARP-1 dominant negative mutant” in each of the following examples) was obtained by RT-PCR using a human spleen mRNA as the temperate. Firstly, a single-stranded cDNA was synthesized by carrying out reverse transcription of the human spleen mRNA (10 ng) using the kit for reverse transcription reaction. Using this single-stranded cDNA as the template, PCR was carried out by a hot start method using a Taq DNA polymerase (LA Taq DNA polymerase; manufactured by Takara Shuzo). The aforementioned PCR was carried out using an oligonucleotide comprising the nucleotide sequence represented by SEQ ID NO:21 as the sense primer, and an oligonucleotide comprising the nucleotide sequence represented by SEQ ID NO:22 as the antisense primer, and firstly carrying out thermal denaturation at 98° C. (1 minute) and then repeating a cycle of 98° C. (15 seconds)/65° C. (30 seconds)/72° C. (6 minutes) 35 times. As a result, a DNA fragment of about 1.2 kbp was amplified. This DNA fragment was cloned into the pCR-TOPO vector using the cloning kit. An insert (cDNA coding for the PARP-1 dominant negative mutant) alone was isolated by digesting the thus obtained plasmid DNA with a restriction enzyme EcoRI and then cloned using a pcDNA3.1(+) plasmid (manufactured by Invitrogen). When nucleotide sequence of the thus obtained clone pcDNA3.1-PARPt was analyzed by the dideoxy terminator method using a DNA sequencer (ABI 3700 DNA Sequencer; manufactured by Applied Biosystems), the sequence represented by SEQ ID NO:19 was obtained. The sequence represented by SEQ ID NO:19 encoded the PARP-1 dominant negative mutant.

Example 15

<Inhibition of Activation of Human LTRPC2 Protein by PARP-1 Dominant Negative Mutant>

In order to examine influence of the PARP-1 dominant negative mutant upon the channel activity of human LTRPC2 protein, expression of the aforementioned protein was induced in an animal cell. Using the expression vectors pcDNA3.1-LTRPC2 and pcDNA3.1-PARPt obtained in Example 1 and Example 14 and a reagent for transfection (LIPOFECTAMINE 2000; manufactured by Invitrogen), transfection of a Chinese hamster ovary-derived CHO dhfr⁻ cell was carried out to induce expression of the human LTRPC2 protein and PARP-1 dominant negative mutant. In this case, the aforementioned operation was carried out in accordance with the protocol attached to the aforementioned reagent for transfection and the known method (reference 1).

Activation of LTRPC2 in the thus obtained transformed cells (1.6×10⁵ cells) was measured using a method measuring efflux of Rb⁺ ion. The method was carried out in the same manner as in Example 6. In this case, however, H₂O₂ (final concentration=0.06%) was used as the PARP activator, and cells in which the LTRPC2 protein alone obtained in Example 5 was expressed were used as the control.

As a result of the measurement, the residual activity of ⁸⁶Rb⁺ in the cells in which the PARP-1 dominant negative mutant was expressed was significantly increased in comparison with the control cells (cells in which the PARP-1 dominant negative mutant was not expressed).

This shows that the PARP activity was inhibited by the PARP-1 dominant negative mutant, and as the result, activation of the human LTRPC2 protein was inhibited and discharge of the intracellular ⁸⁶Rb⁺ into the extracellular moiety was reduced.

In addition, activation of PARP in this case was examined using the method of Example 8. However, H₂O₂ (final concentration=0.06%) was used as the PARP activator, and cells in which the LTRPC2 protein alone obtained in Example 5 was expressed were used as the control. As a result of the measurement, the ³H radioactivity in the cells in which the PARP-1 dominant negative mutant was expressed was reduced to 67% in comparison with the control cells.

It was revealed based on these results that the activation of human LTRPC2 protein by H₂O₂ is inhibited by the PARP-1 dominant negative mutant. In addition to the results obtained by using PARP inhibitors (Example 7), it was confirmed also from the results obtained using the PARP-1 dominant negative mutant (this Example) that the LTRPC2 activation is inhibited by the reduction of PARP activity, and the activity of LTRPC2 therefore is controlled by PARP.

Example 16

<Screening of Human LTRPC2 Protein Inhibitor>

Screening was carried out on a compound capable of inhibiting the ion channel activity of human LTRPC2 protein.

The transfected cells obtained in Example 5 (1.6×10⁵ cells) were incubated at 37° C. for 24 hours in the presence of ⁸⁶RbCl (1 μCi/ml) to effect incorporation of ⁸⁶Rb⁺ into the cells and then washed with physiological saline to remove the ⁸⁶Rb⁺ which was not incorporated into the cells. The thus obtained cells were incubated at room temperature for 30 minutes in physiological saline supplemented with each of various compounds (final concentration=10 μmol/liter) in the presence of H₂O₂ (final concentration=0.06%). In this connection, the same operation was carried out as a control without adding the compound in the presence of H₂O₂.

The respective cells were washed with physiological saline, and then radioactivity of the ⁸⁶Rb⁺ remained in the cells was measured. A compound showing a significantly high residual activity as the thus obtained radioactivity, in comparison with the radioactivity of the control cells (cells to which H₂O₂ alone was added), is a compound which inhibits the channel activity of human LTRPC2 protein. This shows that ⁸⁶Rb⁺ in the cells was not effluxed into the extracellular moiety caused by the inhibition of the activation of human LTRPC2 protein induced by the added compound, or the efflux was reduced.

Since the compounds found in the above are obtained using, as the index, efflux of ⁸⁶Rb⁺ into the extracellular moiety accompanied by the activation of human LTRPC2 protein, the human LTRPC2 protein inhibition activity is not directly detected. More illustratively, a compound showing inhibition activity of PARP contained inside the cells also inhibits activation of the human LTRPC2 protein and inhibits efflux of ⁸⁶Rb⁺ into the extracellular moiety as a result. Accordingly, examination was further carried out on the PARP inhibition activity of the compounds obtained in the aforementioned process.

The transfected cells obtained in Example 5 were incubated at room temperature for 30 minutes in physiological saline supplemented with each of various compounds (final concentration=10 μmol/liter) in the presence of H₂O₂ (final concentration=0.06%). In this connection, the same operation was carried out as a control in the presence of H₂O₂ without adding the compound. Next, the cells after removal of the reaction solution were mixed with 0.2 nCi/μl in final concentration of ³H-NAD and incubated at 37° C. for 40 minutes. Subsequently, the reaction was stopped by adding TCA to the cells to a final concentration of 10% and incubating the mixture at 4° C. for 30 minutes. Thereafter, the un-incorporated ³H-NAD was removed by washing the cells twice with 5% TCA. The thus washed cells were lysed with a solution containing 2% SDS and 0.1% NaOH, and then the ³H radioactivity contained in the cells was measured.

As a result, a compound showing a residual activity similar to the residual radioactivity of the control cells (cells to which H₂O₂ alone was added) was selected. This shows that the added compound is not inhibiting the PARP activity.

As a result of these examinations, a compound which inhibited efflux of ⁸⁶Rb⁺ into the extracellular moiety accompanied by the activation of human LTRPC2 protein but did not inhibit the intracellular PARP activity at the same time was selected. It is considered that this compound is directly inhibiting the activation of human LTRPC2 protein.

Next, in order to verify that the thus found compound is directly inhibiting the human LTRPC2 protein, its influence upon the ion channel activity of human LTRPC2 protein was examined using a technique of electrophysiology. Illustratively, the whole cell current which is generated when ADP ribose (500 μM in final concentration) is administered inside the cells was measured by membrane potential-clamped the transfected cells obtained in the aforementioned Example 5 by the whole cell membrane potential-clamped (whole-cell voltage-clamp) method. A solution containing 145 mmol/L NaCl, 5 mmol/L KCl, 2 mmol/L CaCl₂, 2 mmol/L MgCl₂ and 10 mmol/L HEPES-Na (pH 7.4) was used as the extracellular solution, and a solution containing 150 mmol/L CsCl, 5 mmol/L MgCl₂ and 10 mmol/L HEPES-Cs (pH 7.2) was used as the intracellular solution.

In this case, change in the whole cell current induced by ADP ribose was measured by adding the compound found by the aforementioned process simultaneously with the ADP ribose administration or at the time of the pretreatment. In that case, a compound which reduces change in the whole cell current in comparison with the case of the addition of ADP ribose alone, or completely inhibits induction thereof, is selected. This shows that the added compound directly inhibited the channel activity of human LTRPC2 protein.

By carrying out these examinations, it is possible to find out a compound capable of directly inhibiting the human LTRPC2 protein.

Example 17

<Expression of Human LTRPC2 Protein in Pancreatic Cell and Acceleration of Cell Death of Type I Diabetes Mellitus by Human LTRPC2 Protein>

Function of human LTRPC2 protein in pancreas and participation of human LTRPC2 protein in cell death in a type I diabetes mellitus model were examined. Regarding the cell death of pancreatic cell in type I diabetes mellitus, a cell death by streptozotocin stimulation can be used as its model. In addition, it has been revealed also that PARP is concerned in the process of this cell death (Pieper, A. A. et al., Proc. Natl. Acad. Sci., 96, 3059–3064, 1999).

Using the expression vector pcDNA3.1-LTRPC2 obtained in Example 1 and a reagent for transfection (LIPOFECTAMINE; manufactured by Invitrogen), transfection of MIN6B cell derived from the β cell of the pancreatic islets of Langerhans was carried out to induce expression of the human LTRPC2 protein. In this case, the aforementioned operation was carried out in accordance with the protocol attached to the aforementioned reagent for transfection and the known method (reference 1).

The thus obtained transfected cells were inoculated in 4×10⁴ cells/well portions into a 96 well plate (manufactured by Asahi TechnoGlass), cultured overnight and then incubated at room temperature overnight using a cell culture medium containing 1 mM in final concentration of streptozotocin (manufactured by Sigma Aldrich). After removing the cell culture medium, an MTT Assay was carried out in order to judge life or death of the cells. According to this operation, life or death of the cells can be judged by making use of the reaction of developing blue color through the reduction of MTT into formazan by the respiratory enzyme system in mitochondria in the cells. After 2 hours of culturing by adding an MTT reagent (manufactured by Wako Pure Chemical Industries) to the stimulated cells, formazan was dissolved by adding DMSO to the resulting cells after removing the culture supernatant, and then its absorbance was measured using a spectrophotometer. In this case, the same operation was carried out as a control using cells transfected with the pcDNA3.1 vector alone which does not contain LTRPC2. As a result, the absorbance was significantly reduced (reduced to 93%) in the cells in which LTRPC2 was expressed, in comparison with the cells in which the vector alone was expressed. This shows that cell death was induced via LTRPC2 by the streptozotocin stimulation, and the number of surviving cells was reduced as the result. Based on this result, it was able to confirm that the human LTRPC2 protein accelerates cell death by streptozotocin stimulation. In addition, since the cell death by streptozotocin stimulation is a model of type I diabetes mellitus, it was revealed that the human LTRPC2 has a function to induce cell death by type I diabetes mellitus.

INDUSTRIAL APPLICABILITY

According to the screening method of the present invention, it can screen a substance capable of inhibiting cell death induced by the activation of PARP, particularly a substance which is useful as a therapeutic agent and/or a preventive agent for a disease in which cell death induced by the activation of PARP is concerned (e.g., rheumatoid arthritis, neuronal death at the time of cerebral ischemia, cell death of the heart after myocardial infarction reperfusion, autoimmune destruction of β-cells of pancreatic islets of Langerhans, cell death after shock, or inflammatory reaction by immunocyte death). The screening system of the present invention can be constructed using the LTRPC2 of the present invention and the cell of the present invention.

Though the present invention has been described in the foregoing by specified embodiments, modifications and improvements obvious to those skilled in the art are included in the scope of the invention. 

1. An isolated polypeptide selected from the group consisting of(a) a polypeptide which comprises the amino acid sequence represented by SEQ ID NO:4 or SEQ ID NO:14, and (b) a polypeptide which comprises the amino acid sequence of SEQ ID NO: 4 or SEQ ID NO:14 in which from one to five amino acids are substituted, deleted and/or inserted, and which has an activity to cause permeation of ions.
 2. An isolated polypeptide which consists of the amino acid sequence represented by SEQ ID NO:4.
 3. An isolated polynucleotide which encodes the polypeptide according to claim 1 or
 2. 4. An isolated polypeptide which consists of the amino acid sequence represented by SEQ ID NO:14.
 5. An isolated polynucleotide which encodes the polypeptide according to claim
 4. 6. An expression vector which comprises the polynucleotide according to claim
 3. 7. An expression vector which comprises the polynucleotide according to claim
 5. 8. An isolated host cell which comprises the vector according to claim
 6. 9. An isolated host cell which comprises the vector according to claim
 7. 10. A process for producing the polypeptide according to claim 1 or 2 comprising, culturing an isolated host cell harboring an expression vector that comprises a polynucleotide encoding said polypeptide under conditions promoting expression of said polypeptide and recovering said polypeptide from the cell culture.
 11. A process for producing the polypeptide according to claim 4 comprising, culturing an isolated host cell harboring an expression vector that comprises a polynucleotide encoding said polypeptide under conditions promoting expression of said polypeptide and recovering said polypeptide from the cell culture.
 12. A method for determining whether a compound is an inhibitor of an ion channel, wherein said ion channel is comprised of a polypeptide selected from the group consisting of (a) a polypeptide represented by SEQ ID NO:2, (b) a polypeptide represented by SEQ ID NO:4, (c) a polypeptide with at least 84% sequence homology to the polypeptide represented by SEQ ID NO:2 wherein the polypeptide (c) has an activity to cause permeation of ions, and (d) a polypeptide with at least 84% sequence homology to the polypeptide represented by SEQ ID NO:4, wherein the polypeptide (d) has an activity to cause permeation of ions, said method comprising: (A) contacting a cell expressing (i) PARP-1 and (ii) said polypeptide with (a) a test compound in the presence of an exogenous PARP-1 activator, wherein said PARP-1 activator activates, through PARP, an ion channel comprised of said polypeptide; (B) measuring ion flow through said ion channel; and (C) comparing the ion flow through said ion channel of (B) with the ion flow through an ion channel in a corresponding cell cultured in the presence of said exogenous PARP-1 activator and in the absence of said test compound, wherein when the ion flow through said ion channel is lower in the presence of said test compound than in the absence of said test compound, said test compound is determined to be an inhibitor of an ion channel.
 13. The method of claim 12, wherein said PARP-1 activator is H₂O₂or MNNG.
 14. The method of claim 12, wherein said cell is a eukaryotic cell.
 15. The method of claim 12, wherein flow through said ion channel is measured using a patch-clamp technique, influx or efflux of a radioisotopic ion, efflux of Rb⁺ ion, or an intracellular Ca²⁺ indicator.
 16. The method of claim 12, wherein the cell expresses a polypeptide having the amino acid sequence of SEQ ID NO:2, or SEQ ID NO:4.
 17. A method for determining whether a test compound is an inhibitor of an ion channel, wherein said ion channel is comprised of a polypeptide represented by SEQ ID NO: 14, said method comprising: (A) contacting a cell expressing (i) PARP-1 and (ii) said polypeptide with (a) a test compound in the presence of an exogenous PARP-1 activator, wherein said PARP-1 activator activates, through PARP, an ion channel comprised of said polypeptide; (B) measuring ion flow through said ion channel; and (C) comparing the ion flow through said ion channel of (B) with the ion flow through an ion channel in a corresponding cell cultured in the presence of said exogenous PARP-1 activator and in the absence of said test compound, wherein when the ion flow through said ion channel is lower in the presence of said test compound than in the absence of said test compound, said test compound is determined to be an inhibitor of an ion channel. 